Engines may use boosting devices, such as turbochargers, to increase engine power density. However, engine knock may occur due to increased combustion temperatures. Knock is especially problematic under boosted conditions due to high charge temperatures. The inventors herein have recognized that utilizing an engine system with a split exhaust system, where a first exhaust manifold routes exhaust gas recirculation (EGR) to an intake of the engine, upstream of a compressor of the turbocharger, and where a second exhaust manifold routes exhaust to a turbine of the turbocharger in an exhaust of the engine, may decrease knock and increase engine efficiency. In such an engine system, each cylinder may include two intake valves and two exhaust valves, where a first set of cylinder exhaust valves (e.g., scavenge exhaust valves) exclusively coupled to the first exhaust manifold may be operated at a different timing than a second set of cylinder exhaust valves (e.g., blowdown exhaust valves) exclusively coupled to the second exhaust manifold, thereby isolating a scavenging portion and blowdown portion of exhaust gases. The timing of the first set of cylinder exhaust valves may also be coordinated with a timing of cylinder intake valves to create a positive valve overlap period where fresh intake air (or a mixture of fresh intake air and EGR), referred to as blowthrough, may flow through the cylinders and back to the intake, upstream of the compressor, via an EGR passage coupled to the first exhaust manifold. Blowthrough air may remove residual exhaust gases from within the cylinders (referred to as scavenging). The inventors herein have recognized that by flowing a first portion of the exhaust gas (e.g., higher pressure exhaust) through the turbine and a higher pressure exhaust passage and flowing a second portion of the exhaust gas (e.g., lower pressure exhaust) and blowthrough air to the compressor inlet, combustion temperatures can be reduced while improving the turbine's work efficiency and engine torque.
However, the inventors herein have recognized potential issues with such systems. As one example, under certain operating conditions, such as high engine speeds, increased EGR may flow to the compressor, thereby increasing the compressor speed and temperature. Degradation to the compressor may occur if a gas temperature of gases entering the compressor and/or the speed of the compressor increases above threshold levels. Additionally, condensation may occur at the compressor inlet under colder operating conditions when a temperature at the compressor inlet falls below a threshold level. Condensate may also result in degradation of the compressor. However, the inventors have further realized that controlling engine operating conditions to decrease a likelihood of the above compressor conditions may also result in reduced engine efficiency and fuel economy.
In one example, the issues described above may be addressed by a method, comprising: adjusting, in coordination, an intake valve timing, exhaust valve timing of a first set of exhaust valves, and a position of an exhaust gas recirculation (EGR) valve in an EGR passage in response to a condition at a compressor, where the EGR passage is coupled between an intake passage, upstream of the compressor, and a first exhaust manifold coupled to the first set of exhaust valves. As one example, the condition at the compressor may include one or more of an inlet temperature of the compressor below a threshold inlet temperature, an outlet temperature of the compressor above a threshold outlet temperature, and/or condensation at the compressor inlet. By adjusting the intake valve timing, exhaust valve timing, and the EGR valve position, in coordination with one another, in response to the condition at the compressor, degradation may be reduced while also delivering a desired torque demand and increasing fuel economy. For example, when the compressor is operating within desired operational and temperature ranges, a controller may adjust the intake and exhaust valve timings and EGR valve position based on torque demand and a desired fuel economy level for the current engine operating conditions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.